1. Field of the Invention
This invention relates to a process for preparing alkylene episulfides or episelenides by contacting elemental sulfur or elemental selenium, respectively, with an olefin while irradiating the materials with electromagnetic radiation.
2. Description of the Prior Art
Ethylene and propylene episulfides represent highly reactive and potentially valuable monomers, useful in the production of a variety of functional polymers. Sulfur-containing polymers are most noted for their elastic properties at low temperatures, good weathering ability and resistance to ozone. Thiol terminated liquid polymers have the advantageous capability of in situ curing at room temperature, which has led to applications in sealants and adhesives. Since a large part of the total molecular weight of ethylene episulfide and propylene episulfide is sulfur, these materials could provide a low cost raw material base. Direct routes to ethylene and propylene episulfide from sulfur and the olefins, however, are not available.
The gas phase reaction of ground state S atoms with olefins to form cyclic episulfides has been extensively studied through ultraviolet photolysis of COS and CS.sub.2 (H. E. Gunning and O. P. Strausz, Advances Photochem., 4, 143 (1966)). In the case of ethylene, the reaction has a low (.about.1.6 kcal/mol) activation energy and proceeds with a bimolecular rate constant of 5.times.10.sup.13 cm.sup.3 molecule.sup.-1 sec.sup.-1 (collision probability .about.2.times.10.sup.-3) at 20.degree. C.
Current methods of ethylene episulfide preparation require the reaction of ethylene oxide with CS.sub.2 or COS, the photolysis of COS in the presence of ethylene, or the reaction of ethylene oxide with an aqueous alcoholic solution of a thiocyanate. Major applications of sulfur monomers in large volume polymers have not developed due to the high synthetic cost of the monomers.
In general, the direct interaction of sulfur with olefins results in the formation of a mixture of complex polysulfides. A contributing factor to this complex mixture is the fact that sulfur exists at moderate temperatures in the highly stable S.sub.8 and S.sub.6 polymer forms. Temperatures of at least 600.degree. C. are necessary to form S.sub.2, a moderately reactive form of sulfur, but at these higher temperatures sulfur readily dehydrogenates potential organic reactants. Single S atoms require even higher temperatures (.gtorsim.1700.degree. C.), at which episulfides are unstable.
Only one instance has been reported in which an episulfide was formed by direct reaction of elemental sulfur with an olefin at ambient temperature (F. R. Sharp and T. L. Peppards (Chem. Ind. (1977) 664)). In that case, episulfides of some unsaturated sesquiterpene hydrocarbons (e.g., caryophyllene) were synthesized by the slow reaction of the hydrocarbon with elemental sulfur in daylight. The reaction did not proceed in the dark. The sesquiterpene hydrocarbons contain unusual olefinic linkages with chemical properties different from simple alkenes.